The present invention is directed to hydraulically actuated press brakes. In its most usual form, a hydraulically actuated press brake comprises a stationary bed, a ram mounted above the bed and capable of reciprocating movement toward and away from the bed in a vertical direction, a pair of substantially identical main cylinders operatively connected to the ends of the ram to shift the ram toward and away from the bed, and a source of hydraulic fluid under pressure to actuate the main cylinders. Deflection of the press brake bed and ram when under load against a workpiece constitutes a problem which has plagued the industry from its beginning to the present day. Such deflection of the press brake bed and ram causes their cooperating tool or die-carrying edges to go out of parallelism during the work stroke.
The nature of this problem can most easily be explained by selecting, for example, one of the most common forms of press brake work, i.e., the air bending of a flat piece of metal into an angle. In the most usual instance of such an operation, the top or male die is mounted on the lower edge of the ram. When the ram moves downwardly, the top or male die pushes the workpiece into a lower or female die, mounted on the upper edge of the stationary bed. The inherent problem of bed and ram deflection, occurring as the result of bending load, results in a variation in the bend angle along the length of the bend in the workpiece.
Prior art workers have made numerous attempts to overcome this problem. A common practice is to shim one of the dies (usually the female die), or the filler block thereunder, so that the dies are parallel in the loaded condition. This solution is effective but less than ideal. It is a trial-and-error process and hence very time consuming. In today's factory, the trend to small lot sizes, makes this solution unacceptable.
Another approach has been to crown the tool or die carrying edges of the bed and ram, so that these edges are purposely not parallel in the unloaded condition. They are closer together in the center than at the ends. The amount of this crown is carefully controlled so that the bed and ram are deflected to a parallel position when subjected to a load equal to some predetermined fraction (say 2/3) of the machine's capacity distributed uniformily over the nominal length. In bending operations wherein the load is equal to (or substantially equal to) this predetermined fraction of the machine's capacity, the male die penetration into the female die is constant and the bend angle is uniform along its length. For bending operations which depart from these predetermined conditions, this solution does not work well.
The present invention is based upon the discovery of means which may be applied to the bed or the ram to give the bed or ram a variable, proportional crown which will automatically compensate for both bed and ram deflection for an applied load of any magnitude and length. The structure of the present invention causes the bed and ram tool or die-carrying edges to remain substantially parallel in the loaded condition, as will be described hereinafter.